It is well known that vinyl chloride polymers, including homopolymers of vinyl chloride, and copolymers of vinyl chloride with other monomers copolymerizable therewith, have inherently low-impact properties. The burgeoning demand for polyvinyl chloride (PVC) articles in particular, and the fact that impact-deficient vinyl chloride polymers lend themselves to being transformed to impact-resistant polymers by the addition of impact modifiers, has led to the expenditure of great effort to develop better impact modified vinyl chloride compositions. This effort is especially directed to rigid-PVC applications where acceptable impact strength is critical. Such applications include the manufacture of exterior structural products, rigid panels, pipe and conduit, injection-molded and thermoformed industrial parts, appliance housings, and various types of containers both large and small.
Elastomeric additives which reduce the brittle temperature of vinyl chloride polymers, are the most commonly used impact modifiers and include conjugated diene homo and copolymers or, acrylic polymers. Particularly preferred are graft copolymers containing butadiene which generally offer excellent impact improvement because of their low glass-transition temperatures (T.sub.g).
Other impact modifiers include chlorinated polyethylene (CPE), ethylene-vinyl acetate copolymers (EVA), acrylate-methacrylate (all acrylic) polymers, grafted elastomers based on vinyl chloride (VC), nitrile rubbers (NBR), styrene-butadiene rubbers (SBR) and stearic acid coated calcium carbonate. These impact modifiers are conventionally used in a substantial amount, that is, an amount sufficient to provide at least a 20% increase in Izod impact strength of a non-impact modified vinyl chloride polymer matrix. Since, in this invention, the conventional impact modifiers are used in combination with block copolymer impact modifiers, the conventional impact modifiers are referred to herein as "co-modifiers".
Graft copolymers such as acrylonitrile-butadiene-styrene (ABS), or methyl-methacrylate-butadiene-styrene (MBS) have become the major impact modifiers in commercial use because they retain sufficient elastomeric qualities for good impact behavior, and also sustain the product clarity essential to many packaging applications. Such graft copolymers are preferably overpolymerized graft copolymers. There was no reason to expect that a block copolymer of any kind, and more particularly a block copolymer selected from a linear diblock or polyblock copolymer and a radial teleblock copolymer of a vinyl aromatic (S) and a conjugated diene (D), defined hereinafter, should or could synergistically boost the impact effectiveness of the co-modifier in a conventionally impact-modified vinyl chloride polymer. Yet, it does.
It is because of this boosting power of a SD and/or a SDS block copolymer, namely, to boost the impact resistance attributable to conventional co-modifiers, that the block copolymer is referred to herein as an "ultra-modifier". A preferred ultra-modifier is selected from a styrene-butadiene (SB) diblock, and a linear polyblock or radial teleblock styrene-butadiene-styrene (SBS) copolymer, and in this invention, the ultra-modifier must be used in combination with a co-modifier, provided the co-modifier, used in the absence of ultra-modifier, produces substantial impact improvement.
Styrene-diene-styrene (SDS) block copolymers with styrene blocks on the ends of the polymer molecule are known to produce strong elastomeric materials without vulcanization. When used as a sole impact modifier, they are known to produce marginal impact resistance in vinyl chloride polymers (Potepalova S.N., Malyshev L.N. and Savel 'ev A.P., Plast. Massy. 8 41, 1973). In view of such marginal impact improvement attributable to these block copolymers when used alone, it is surprising that, when combined with conventional co-modifiers, these block copolymers synergistically boost the impact properties of PVC.
U.S. Pat. No. 3,825,622 to Robeson L.M. et al., discloses blends of (a) vinyl chloride polymer with (b) from about 40 to about 85 parts by weight, per 100 parts of vinyl chloride polymer, of a graft copolymer of a lactone grafted onto a reactive polymer of ethylenically unsaturated monomers, and (c) about 7 to about 40%, based on the weight of lactone copolymer, of a SDS block copolymer. There is no suggestion that the lactone graft copolymer provides any desirable impact modification and it does not. The reference states the lactone copolymer is a desirable plasticizer; however because the plasticized blend of vinyl chloride resin and lactone graft copolymer does not provide sufficient impact resistance, the reference teaches adding an SDS block copolymer. More specifically it is evident that vinyl chloride resin and graft copolymer plasticizer exist as a single phase characteristic of a plasticized vinyl chloride resin. In contrast, any impact modifier, and particularly a graft copolymer impact modifier, exists in an impact modified vinyl chloride resin as a separate phase, whether the resin is plasticized or not. The existence of a rubbery phase dispersed in a continuous vinyl chloride resin phase critically distinguishes an impact modified vinyl chloride resin from one that is only plasticized. The reference teaches a plasticized vinyl chloride resin which is impact modified with SDS block copolymer present as the sole impact modifier to yield a flexible film with good impact properties as determined by the Masland cold crack test (a modified ASTM D-1790 test). The composition containing the SDS, which is not a rigid composition, then exhibits impact resistance, because the SDS is peculiarly compatible with the vinyl chloride polymer lactone graft copolymer blend.
The effect of the ultra-modifier is best demonstrated when the co-modifier consists of a rubbery backbone having graft copolymerized thereupon a hard resin which forms a hard shell (hence referred to as "hard shell type" resins). At least about 80% of the primary particles of the hard shell resin are in a size range from about 500 A to about 5000 A, generally either in a bimodal or a broad normal distribution of particle sizes. Known such graft copolymer impact modifiers for PVC typically include a rubbery base chain onto which is graft copolymerized homopolymers or copolymers of vinyl chloride, methyl acrylate, alkyl methacrylates, styrenes, substituted styrenes, acrylonitrile, methacrylonitrile, and the like. Other impact modifiers which are elastomeric additives also contribute a rubbery phase of discrete particles or agglomerates in the vinyl chloride resin phase, and together with an ultra-modifier diblock (SD) or polyblock (SDS) copolymer, provide a synergistic improvement of impact properties.
U.S. Pat. No. 3,383,435 discloses a co-modifier of the "hard shell" type in which styrene is homopolymerized into side chains onto a butadiene rubber backbone. Other hard-shell type co-modifiers are ABS resins disclosed in U.S. Pat. Nos. 2,802,808 and 3,238,275; PMMA/PBA resins disclosed in U.S. Pat. No. 3,655,825; MBS resins disclosed in U.S. Pat. No. 3,671,610; PVC/PBA resins in which vinyl chloride is overpolymerized on butylacrylate rubber; and the like.
The foregoing references disclose graft copolymers of the "hard shell" type used as impact modifiers in PVC, which hard shell type is generally characterized by a T.sub.g higher than 70.degree. C., and preferably higher than 85.degree. C. Though much theoretical work has been done to predict impact performance of various compounds, there is nothing to suggest that a hard shell type graft copolymer co-modifier be blended with SD or SDS block copolymer to increase the impact strength of PVC containing the co-modifier.
Though an SDS block copolymer has been used as the only impact modifier in vinyl chloride polymers, it is difficult to blend the block copolymer into the vinyl chloride polymer. For example, S. N. Potepalova et al in Plast. Massy. 8,41 (1973) reported that prolonged mixing was required to get a marginal improvement in impact strength when SBS was used as the sole impact modifier in PVC. There is no suggestion that SBS might behave quite differently, when used with a conventional impact modifier in vinyl chloride polymer resins, and especially, none that it may act as an ultra-modifier. There is no reason to expect that both SDS and DSD type block copolymers, whether linear or radial, diblock or triblock, might provide desirable impact properties only if a conventional impact modifier was also present.